1. Field of the Invention
The present invention pertains to improvements in drilling mud-cement compositions for cementing oil and gas wells. The compositions are characterized by improved setting at low temperatures and resistance to strength retrogression at temperatures above 230.degree.-250.degree. F.
2. Description of Prior Art
The general procedure of drilling an oil or gas well includes drilling a borehole using a drilling mud. Subsequent to drilling the borehole, casing is run into the well preparatory to placing a cement slurry in the annulus between the outside of the casing and the borehole wall. Wells with temperatures above 230.degree.-250.degree. F. present special cementing problems. For example, converted drilling fluid compositions known as C-Mix suggested by R. E. Wyant's U.S. Pat. No. 3,499,491 are particularly temperature sensitive. In other words, if wellbore temperatures exceed 230.degree.-250.degree. F., the cement compositions (C-Mix) or any converted drilling fluids suggested by Wilson in U.S. Pat. Nos. 4,883,125 and 4,176,720 have a tendency to undergo thermal strength retrogression. Since the C-Mix composition contains a substantial mount of Portland cement, the set C-Mix has a tendency to undergo strength retrogression at temperatures above approximately 230.degree.-250.degree. F.
Thermal strength retrogression is a particular problem in cementing any deep, hot wells or wells associated with thermal recovery processes. For example, the Belridge field in California has two major producing zones, the Tulare Sands and the Diatomitc/Brown Shale. The Tulare zone has been extensively steam flooded for several years such that temperatures in producing intervals range from 250.degree. F. to 400.degree. F. The Diatomitc/Brown Shale formation lies beneath the Tulare Sands and extends from 700 feet to 4,000 feet. Since C-Mix undergoes severe strength retrogression at temperatures 230.degree.-250.degree. F. it cannot be used in cementing wells in this field.
Wells with low bottom hole temperatures also present special cementing problems. For example, wells in the North Hobbs field, New Mexico, have low bottom hole temperatures, e.g., approximately 100.degree. F. at a depth of 4,000 feet. In addition, these wells are normally drilled with 10.5 lb/gal salt saturated muds. A low temperature, salt tolerant mud-cement formulation is needed to cement these wells. Another example for low temperature wells are those deep-water wells in the Gulf of Mexico. Mud-lime temperatures range between 40.degree. F. and 60.degree. F. Surface casing cements for these wells must be activated at low temperatures.
Accordingly, the present invention is directed to overcoming the above-noted problems in the art and provides a solution as more particularly described hereinafter.